In Nô-theatre - which I take to indicate how she was imagined in pre-Edo period times - we usually find her with round eyes and slightly reddish color.
During the Edo-period she seems to have been portrayed in two very different ways. Although there are intermediate forms, she either tended to be a witchlike ogre (oni-baba), as on this illustration by Toriyama Sekien (1712-1788):
Or else she is an affectionate and loving mother, as on the paintings below by Utamarô (1753-1806) and Kawanabe Kyôsai (1831-1889).
The child is Kintarô, a boy of superhuman strength who according to legend was raised by Yamauba in the mountains where he wrestled with bears (note the bear on Kyôsai's painting).
Earlier I wrote that superhuman beings in Japan were often portrayed with round eyes. They were also often associated with the color red (as tengu, oni, Bodhidharma etc). These traits recur in how Yamauba was portrayed in Nô-plays.
Strikingly, however, she looks almost "human" whenever she is portrayed as a mother. Her eyes are narrow and her color is white or pale, just as the color of women on Edo period paintings. The only thing that marks her off as associated with superhuman power is her long and disheveled hair
We also find the Yamauba-as-mother motive in these sweet figurines from the late Edo-period, where even the hair is tied up and her human character even more emphasized:
Kintarô, by contrast, has retained the attributes of the supernatural - round eyes and red color. The result is a striking combination in both the paintings and the figurines of white and red, narrow eyes and round eyes, human appearance and superhuman appearance.
In the motherly figure of Yamauba, we recognize an image of an ideal mother, not so far removed from Kishiboshin, the Buddhist protector of children. I suspect there must have been a tendency in folk belief to fuse Yamauba with Kishiboshin.
The legends about Kishiboshin make this similarity even more striking. For simplicity's sake, let me quote Wikipedia:
Originally, Kishimojin/ Hariti was a cannibalistic demon. She had hundreds of children whom she loved and doted upon, but to feed them, she abducted and killed the children of others. The bereaved mothers of her victims pleaded to Śākyamuni Buddha to save them. Śākyamuni stole Aiji, youngest of Kishimojin's sons, and hid him under his rice bowl. Kishimojin desperately searched for her missing son throughout the universe. Finally, she pleaded with Shakyamuni for help. The Buddha pointed out that she was suffering because she lost one of hundreds of children, and asked if she could imagine the suffering of parents whose only child had been devoured. She replied contritely that their suffering must be many times greater than hers, and vowed to protect all children.Just like Yamauba, Kishiboshin was once an ogre or demon, but, again like Yamauba, she is depicted in human form as soon as her motherly aspect is foregrounded.
Why are Yamauba and Kishiboshin "humanized" as soon as they are portrayed as mothers? Probably because it was impossible to portay ideal mothers in any other way. In order to portray them as good mothers they had to be portrayed as human.